This invention relates generally to assaying of isotopic gaseous separation systems and more particularly to a uranium isotope separation monitoring method and system for determining the uranium-235 (.sup.235 U) enrichment in a uranium separation process, wherein the uranium is in the form of uranium hexafluoride (UF.sub.6) in the gaseous state, without the knowledge of the operating parameters of the gas system. This invention is a result of a contract with the U.S. Department of Energy.
In certain applications, such as safeguards inspection and monitoring of a uranium isotope separation plant wherein the uranium is in the form of UF.sub.6 in the gaseous state, it is desirable to audit the .sup.235 U enrichment factor without access to the process plant operating parameters. While a safeguards inspection team could conduct a .sup.235 U audit more accurately by obtaining samples from various points in the process stream and isotopically analyzing them, permission to do this might not be granted; therefore, a method for determining the .sup.235 U enrichment of the UF.sub.6 gas inside the process equipment from measurements made outside the equipment must be available. Particularly in the case of a centrifuge cascade, the UF.sub.6 is circulated at subatmospheric pressure to maintain the UF.sub.6 in the gaseous state at ambient temperatures. Because of the low pressure of the gas in the cascade piping and since in all locations the .sup.238 UF.sub.6 isotope may comprise more than 95% of the gaseous mixture, only a few gamma rays of 185 keV energy are observed due to the decay of the .sup.235 UF.sub.6 component per unit length of pipe over a particular area. To generate enough signal for a statistically valid measurement of the .sup.235 U content in the gas mixture would require large scintillation crystals with large shields of sophisticated design. Further, the pressure would need to be known in order to infer the total quantity of gas being observed.
Measurement of the .sup.235 U content, without knowledge of total UF.sub.6 present or other system operating conditions, may be carried out by first obtaining a direct measurement of the 185 keV .sup.235 U gamma radiation in a section of pipe, measuring the gamma transmission to determine attenuation by the pipe and total UF.sub.6 combined, making a correction for the attenuation by the pipe wall to provide an estimate of absorption due to UF.sub.6 alone and calculation of total UF.sub.6 in the pipe based on this attenuation effect. The ratio of .sup.235 U-to-total uranium would then be the necessary enrichment factor. The above procedure is a laborious process requiring elaborate new shielding designs and calibrations for every different pipe diameter. The received radiation from the .sup.235 U is strongly affected by the geometrical configuration and unfortunately is best determined by the laborious experimental mock-up method.